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Monitor (synchronization)

Object or module in concurrent programming

  • 54 min read
  • 3 chapters

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Through Object (computer science), Class (programming) and Mutual exclusion

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Another definition of monitor is a thread-safe object, class, or module that contains and uses a mutex to safely allow access to its methods or variables by more than one thread. The defining characteristic of a monitor is that its methods are executed with mutual exclusion: at each point in time, at most one thread may be executing any of the monitor's methods. By using one or more condition variables it can also provide the ability for threads to wait on a certain condition (thus using the first definition of a "monitor"). For the rest of this article, this sense of "monitor" will be referred to as a "thread-safe object/class/module."

Monitors were invented by Per Brinch Hansen and C. A. R. Hoare, and were first implemented in Brinch Hansen's Concurrent Pascal language.

Mutual exclusion

While a thread is executing a method of a thread-safe object, it is said to occupy the object, by holding its mutex (lock). Thread-safe objects are implemented to enforce that at any point in time, at most one thread may occupy the object. The lock, which is initially unlocked, is locked at the start of each public method, and is unlocked at each return from each public method.

Upon calling one of the methods, a thread must wait until no other thread is executing any of the thread-safe object's methods before starting execution of its method. Note that without this mutual exclusion, two threads could cause data races and logical errors. For example, two threads withdrawing 1000 from the account could both return true, while causing the balance to drop by only 1000, as follows: first, both threads fetch the current balance, find it greater than 1000, and subtract 1000 from it; then, both threads store the balance and return.

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